U.S. patent application number 10/890090 was filed with the patent office on 2005-01-20 for magnetic recording and reproduction method.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Matsumoto, Ayako, Noguchi, Hitoshi, Saito, Shinji.
Application Number | 20050013046 10/890090 |
Document ID | / |
Family ID | 33475544 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050013046 |
Kind Code |
A1 |
Noguchi, Hitoshi ; et
al. |
January 20, 2005 |
Magnetic recording and reproduction method
Abstract
A magnetic recording and reproduction method comprising
recording or reproducing a track of a rotating magnetic disk by
means of a magnetic head, wherein the magnetic disk comprises a
support and a magnetic layer containing ferromagnetic powder and a
binder; the track has a breadth of 2 .mu.m or less; and a
frictional force between the magnetic layer and the magnetic head
at innermost peripheral area of a recording region of the magnetic
disk is 30 mN or less, a frictional force between the magnetic
layer and a magnetic head at outermost peripheral area of a
recording region of the magnetic disk is 20 mN or less, and a ratio
of the frictional force at the innermost peripheral area to the
frictional force at the outermost peripheral area is from 1.0 to
4.0.
Inventors: |
Noguchi, Hitoshi; (Kanagawa,
JP) ; Matsumoto, Ayako; (Kanagawa, JP) ;
Saito, Shinji; (Kanagawa, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
33475544 |
Appl. No.: |
10/890090 |
Filed: |
July 14, 2004 |
Current U.S.
Class: |
360/135 ;
G9B/5.024; G9B/5.025; G9B/5.202; G9B/5.293 |
Current CPC
Class: |
G11B 2005/001 20130101;
G11B 5/016 20130101; G11B 5/82 20130101; G11B 2005/0016 20130101;
G11B 5/58 20130101; G11B 5/012 20130101 |
Class at
Publication: |
360/135 |
International
Class: |
G11B 005/82 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 14, 2003 |
JP |
P.2003-274091 |
Claims
What is claimed is:
1. A magnetic recording and reproduction method comprising
recording or reproducing a track of a rotating magnetic disk by
means of a magnetic head, wherein the magnetic disk comprises a
support and a magnetic layer containing ferromagnetic powder and a
binder; the track has a breadth of 2 .mu.m or less; and a
frictional force between the magnetic layer and the magnetic head
at innermost peripheral area of a recording region of the magnetic
disk is 30 mN or less, a frictional force between the magnetic
layer and a magnetic head at outermost peripheral area of a
recording region of the magnetic disk is 20 mN or less, and a ratio
of the frictional force at the innermost peripheral area to the
frictional force at the outermost peripheral area is from 1.0 to
4.0.
2. The method according to claim 1, wherein the frictional force at
the innermost peripheral area is 20 mN or less, the frictional
force at the outermost peripheral area is 10 mN or less, and the
ratio of the frictional force at the innermost peripheral area to
the frictional force at the outermost peripheral area is from 1.0
to 2.0.
3. The method according to claim 1, wherein the ferromagnetic
powder is hexagonal ferrite powder.
4. The method according to claim 1, wherein the magnetic head is an
anisotropic magnetoresistive head.
5. The method according to claim 1, wherein the magnetic disk
rotates at 1,800 rpm or more.
6. The method according to claim 1, wherein the magnetic disk
rotates at 7,800 rpm or less.
7. The method according to claim 1, wherein the magnetic disk
further comprises a lower layer so that the support, the lower
layer and the magnetic layer are in this order, wherein the lower
layer contains nonmagnetic powder and a binder.
8. The method according to claim 1, wherein the magnetic disk
comprises two magnetic layers, so that one of the two magnetic
layers, the support, and other of the two magnetic layers are in
this order.
9. The method according to claim 1, wherein the magnetic layer
contains a higher fatty acid ester and a long chain fatty acid.
10. The method according to claim 1, wherein the magnetic layer
further contains diamond particles as an abrasive.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a magnetic disc, in
particular a magnetic disc using an Anisotropic Magnetoresistive
(AMR) HEAD.
BACKGROUND OF THE INVENTION
[0002] As recording media for recording data of, e.g., personal
computers, 2HD flexible discs (hereinafter referred to as
"2HD-FD"), which are flexible magnetic discs, are now widely used,
and a disc drive for 2HD-FD is carried in many personal computers
as a standardized equipment. However, in recent years, the amount
of data to be dealt with greatly increases, and the recording
capacity of 2HD-FD is no longer sufficient in many cases.
Accordingly, the increase in capacity of a flexible magnetic disc
capable of easily handling is desired.
[0003] To meet the requirement, various techniques for the purpose
of increasing the recording capacity of magnetic recording media
have been developed and flexible magnetic discs far exceeding the
recording capacity of 2HD-FD, e.g., magnetic discs having a
recording capacity of 100 MB or higher, have been put to practical
use. To cope with high density recording of data and data transfer
at a high speed, recording and reproduction of these high capacity
magnetic discs are performed with a magnetic disc apparatus using a
magnetic head.
[0004] As is already known, there are cases where very important
data (information) are recorded on magnetic discs, and recording
and reproduction of data are generally performed repeatedly.
Accordingly, it is of course required of high capacity magnetic
discs to have excellent durability. Further, in recording and
reproducing data, the load applied to the magnetic head of a
magnetic recording apparatus is preferably smaller. For these
requirements, coming of a magnetic disc having more excellent
characteristics is required.
[0005] When a magnetic disc is revolved at a high speed and a
magnetic head is run for seeking the surface of the disc for the
prescribed time (e.g., from 200 to 300 hours in high temperature
environment), there is a case where the magnetic disc takes a
scratch and data cannot be recorded or reproduced, which leads to
the occurrence of an error. One cause of the occurrence of
scratches on a magnetic disc is the evaporation of a lubricant due
to running and the reduction of a lubricating function.
[0006] It is disclosed in JP-A-2002-74649 (The term "JP-A" as used
herein refers to an "unexamined published Japanese patent
application".) that in the magnetic disc disclosed in the patent
comprising a magnetic layer having provided on an intermediate
layer, an appropriate amount of alkylamine lubricant is supplied to
the surface of the magnetic disc without being adsorbed onto the
nonmagnetic powder contained in the intermediate layer, so that the
reduction of a friction coefficient can be realized without
deteriorating electromagnetic characteristics and excellent
reliability of the magnetic disc can be ensured.
[0007] Further, it is shown in EXAMPLES of JP-A-2003-16638 that the
durability of a medium can be improved by reducing the frictional
force applied to a magnetic head. However, it is known that a
frictional force increases when the relative velocity between a
head and a medium lowers, so that it is necessary to secure proper
frictional characteristics in a wide range of relative velocity for
ensuring stable frictional characteristics from the inside diameter
to the outside diameter of a magnetic disc. Further, it has been
known that when the value of frictional force is greatly different
at the radius position, the attitude of a head changes, the spacing
loss between a head and a disc becomes great and electromagnetic
characteristics lower. This tendency is conspicuous when recording
density is high, so that the improvement of durability was out of
the question in the recording/reproducing system as attained in
JP-A-2003-16638 having a track breadth in reproduction of 5 .mu.m
or more.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide a magnetic
disc for high density recording capable of obtaining stable SNR
throughout the recording area and excellent in running
durability.
[0009] The above object of the invention can be solved by the
following means.
[0010] (1) A magnetic recording and reproduction method comprising
recording or reproducing a predetermined track of a rotating
magnetic disk by means of a magnetic head, wherein the magnetic
disk comprises a support and a magnetic layer containing
ferromagnetic powder and a binder; the track has a breadth of 2
.mu.m or less; and a frictional force between the magnetic layer
and the magnetic head at innermost peripheral area of a recording
region of the magnetic disk is 30 mN or less, a frictional force
between the magnetic layer and a magnetic head at outermost
peripheral area of a recording region of the magnetic disk is 20 mN
or less, and a ratio of the frictional force at the innermost
peripheral area to the frictional force at the outermost peripheral
area is from 1.0 to 4.0.
[0011] (2) The method according to the item (1) above, wherein the
frictional force at the innermost peripheral area is 20 mN or less,
the frictional force at the outermost peripheral area is 10 mN or
less, and the ratio of the frictional force at the innermost
peripheral area to the frictional force at the outermost peripheral
area is from 1.0 to 2.0.
[0012] (3) The method according to the item (1) above, wherein the
ferromagnetic powder is hexagonal ferrite powder.
[0013] (4) The method according to the item (1) above, wherein the
magnetic head is an Anisotropic Magnetoresistive (AMR) Head.
[0014] (5) The method according to the item (1) above, wherein the
magnetic disk rotates at 1,800 rpm or more.
[0015] (6) The method according to the item (1) above, wherein the
magnetic disk rotates at 7,800 rpm or less.
[0016] (7) The method according to the item (1) above, wherein the
magnetic disk further comprises a lower layer so that the support,
the lower layer and the magnetic layer are in this order, wherein
the lower layer contains nonmagnetic powder and a binder.
[0017] (8) The method according to the item (1) above, wherein the
magnetic disk comprises two magnetic layers, so that one of the two
magnetic layers, the support, and other of the two magnetic layers
are in this order.
[0018] (9) The method according to the item (1) above, wherein the
magnetic layer contains a higher fatty acid ester and a long chain
fatty acid.
[0019] (10) The method according to the item (1) above, wherein the
magnetic layer further contains diamond particles as an
abrasive.
[0020] The durability of a medium in the invention can be greatly
improved by making the frictional force applied to a head at the
innermost diameter to the outermost diameter low, and at the same
time high SNR can be obtained throughout the inner periphery and
the outer periphery even with a high track density of a track
breadth of 2 .mu.m or less by controlling the ratio of frictional
force applied to a head at the innermost periphery and the
outermost periphery.
DETAILED DESCRIPTION OF THE INVENTION
[0021] In a magnetic disc used in recording/reproducing system
using a track breadth at the time of reproduction of 2 .mu.m or
less, the invention restricts the frictional forces (F) between a
magnetic layer and a magnetic head, i.e., the frictional force at
the innermost peripheral area of a recording region (Fin) to 30 mN
or less, the frictional force at the outermost peripheral area of a
recording region (Fout) to 20 mN or less, and the value of Fin/Fout
to 1.0 to 4.0.
[0022] By the restriction of the frictional forces as above, the
present invention can provide a magnetic disc capable of high
density recording, showing SNR stable throughout the recording
region, and excellent in running durability.
[0023] Frictional force (Fin) is a frictional force that can be
obtained by rotating a magnetic disc, arranging a magnetic head in
the rotary tangential direction of the magnetic disc and at the
innermost peripheral part of a recording region, and detecting the
force applied to the magnetic head with a micro load cell.
Frictional force (Fin) is restricted to 30 mN or less, preferably
20 mN or less.
[0024] Frictional force (Fout) is a frictional force obtained by
the same manner as in frictional force (Fin) except that a magnetic
head is arranged at the outermost peripheral part of a recording
region. Frictional force (Fout) is restricted to 20 mN or less,
preferably 10 mN or less.
[0025] The rotary speed of a magnetic disc in measuring the
frictional forces is not particularly restricted so long as the
speed is within the range of the ordinary speed in the magnetic
disc apparatus where the magnetic disc is used, is preferably 1,800
rpm or more, is preferably 7,800 rpm or less, and more preferably
from 1,800 to 7,200 rpm.
[0026] Fin/Fout is from 1.0 to 4.0, preferably from 1.0 to 2.0.
[0027] The frictional forces in the invention are values which are
satisfied in every environment of 23.degree. C. 50% RH.
[0028] The measuring methods of the frictional forces are disclosed
in detail in JP-A-2003-16638 and others.
[0029] A magnetic head for use in the present invention is a
thin-film head including an AMR element. The head part of the
magnetic head is brought upon the surface of a magnetic disc with a
weak force by spring force such as gimbal, and at the same time
flies by the air current caused by high rotation of the magnetic
disc to maintain a slight clearance between. The head is completely
flying at the front rotating part of the head, the flying amount at
the rear end is smaller and the head comes in contact with the disc
sometimes. Accordingly, frictional force (F) is caused between the
magnetic disc surface and the magnetic head.
[0030] It is presumed that the magnitude of frictional force (F)
varies by the surface states of a magnetic disc, e.g., the
unevenness and the hardness of the surface of a magnetic disc. The
smaller the frictional force, the lower is the probability of the
damage of a magnetic disc surface and a magnetic head.
[0031] In the invention, as described above, electromagnetic
characteristics can be reconciled with running durability by
controlling frictional force (Fin), frictional force (Fout) and
Fin/Fout.
[0032] A magnetic disc in the present invention can maintain a
lubricating function in long time use, has high durability and
excellent magnetic characteristics. When a frictional force
increases, there are cases where a tracking error is liable to
occur and data cannot be recorded or reproduced, but these
drawbacks can be improved by suppressing a frictional force and the
reliability of a magnetic disc can be increased.
[0033] A magnetic disc in the invention is controlled of the
surface states such as the unevenness and hardness of the surface
and lubricant so that the prescribed frictional force (Fin),
frictional force (Fout) and Fin/Fout can be obtained.
[0034] As the means for controlling the surface states of a
magnetic disc, the selection of the binders contained in a magnetic
layer (Tg, physical strength and the like), the selection of
powders (the structures, configurations and sizes of magnetic
powders, abrasives, carbon blacks and the like), the selection of
lubricants (kinds and addition amounts), the selection of solvents,
the selection of dispersing methods, and the selection of the
conditions of calendering treatment are exemplified.
[0035] Since the peripheral speed is different at the inner and
outer circles of the disc, there are, roughly speaking, the
following methods of controlling the ratio of Fin/Fout.
[0036] (1) By restricting the recording region, the system is
designed so that the ratio of Fin/Fout falls within the defined
range.
[0037] (2) The disc rotation for the recording and reproduction at
the inner periphery is altered from that at the outer
periphery.
[0038] (3) Such a disc is produced that exhibits a frictional force
weakly dependent on relative speed.
[0039] In particular, to achieve the above (3), the following
methods may be adopted.
[0040] To reduce the frictional force in the outer periphery region
(a high relative speed region), it is preferred to adopt a higher
fatty acid ester exhibiting fluid lubrication. Specifically, those
higher fatty acid esters described in `Detailed Description in the
Invention` are preferably used. In particular, by increasing the
addition level to the magnetic layer and the non-magnetic layer,
the frictional force at the high relative speed region (Fout) can
be suppressed. But, when the addition level is raised too high, the
frictional force at the low relative speed region (Fin) unfavorably
increases. The specific addition level is 1 to 20% by weight, and
preferably 3 to 15% by weight for the magnetic material in the
magnetic layer and the non-magnetic powder in the non-magnetic
lower layer. In particular, it is preferred that the addition level
for the magnetic layer is set lower than that for the non-magnetic
lower layer.
[0041] In addition, reduction of the frictional force in a low
relative speed region (Fin) can be achieved by making the particle
size of the non-magnetic powder added to the magnetic layer large,
relaxing the degree of dispersion, etc. Specifically, an abrasive
or carbon black having a large mean particle diameter may be
incorporated in the magnetic layer, or the abrasive or carbon black
to be incorporated and the magnetic material may be dispersed
separately, followed by blending both dispersions together,
etc.
[0042] A magnetic disc according to the present invention is
described in detail below.
[0043] Magnetic Layer:
[0044] A magnetic disc in the invention is generally provided with
a magnetic layer on both sides of a support, but may be provided on
one side.
[0045] A magnetic layer provided on one side of a support may be a
monolayer or may be multilayers each having different composition.
It is preferred in the invention to provide a substantially
nonmagnetic lower layer (also referred to as "a nonmagnetic layer"
or "a lower layer") between a support and a magnetic layer by a
wet-on-wet or wet-on-dry method. A magnetic layer is referred to as
an upper layer or an upper magnetic layer.
[0046] Ferromagnetic powders for use in a magnetic layer are not
especially restricted, but ferromagnetic metal powders and
hexagonal ferrite powders are preferably used, and hexagonal
ferrite powders are particularly preferred.
[0047] Ferromagnetic metal powders are not particularly limited so
long as they contain .alpha.-Fe as a main component (including
alloys). These ferromagnetic powders may contain, in addition to
the prescribed atoms, e.g., Al, Si, S, Ca, Ti, V, Cr, Cu, Y, Mo,
Rh, Pd, Ag, Sn, Ba, Ta, W, Au, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn,
Ni, Sr and B. Ferromagnetic powders containing at least one of Al,
Si, Ca, Y, Ba, La, Nd, Co, Ni and B in addition to .alpha.-Fe are
preferred, and those containing Co, Al, Y and Nd are particularly
preferred. Further in detail, ferromagnetic powders containing from
10 to 50 atomic % of Co, from 2 to 20 atomic % of Al, and from 3 to
20 atomic % of Y and Nd, respectively based on Fe, are
preferred.
[0048] For bringing out the maximum of aptitude in high density
region, ferromagnetic metal powders excellent in high output, high
dispersibility and orientation are used in the invention. That is,
high output and high durability can be attained with ferromagnetic
metal powders comprising hyper-fine particles, particularly having
an average long axis length of from 30 to 65 nm, having a
crystallite size of from 80 to 140 .ANG., containing a great amount
of Co, and containing Al and Y compounds as sintering inhibitors.
In addition, it is necessary that these ferromagnetic metal powders
be excellent in particle size distribution, so that they preferably
have a variation coefficient of long axis length (standard
deviation of long axis length/average long axis length) of from 0
to 30%, an average acicular ratio of from 3.5 to 7.5, a coercive
force of from 143 to 223 kA/m, a saturation magnetization of from
85 to 125 A.multidot.m.sup.2/kg, and a specific surface area by a
BET method (S.sub.BET) Of from 45 to 120 m.sup.2/g. These particles
can be obtained by the methods disclosed in JP-A-9-22522,
JP-A-9-106535, JP-A-6-340426, and JP-A-11-100213, and combinations
of these methods.
[0049] For achieving high density recording, the coercive force of
ferromagnetic powders is preferably high, e.g., from 143 to 223
kA/m, although it is dependent upon the performance of the
recording head to be used. With the increase of a coercive force,
overwriting of signals becomes a problem. Since the coercive force
of ferromagnetic metal powders primarily originates in the
anisotropy of configuration, the variation coefficient of
configuration is preferably small.
[0050] As hexagonal ferrite magnetic powders, magnetoplumbite
structural (M-type) hexagonal ferrites are preferably used, e.g.,
barium ferrite, strontium ferrite, lead ferrite, calcium ferrite,
and various substitution products of these ferrites are
exemplified. These hexagonal ferrite powders may contain, besides
the prescribed atoms, the following atoms, e.g., Al, Si, S, Sc, Ti,
V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Ba, Ta, W, Re, Au, Pb, Bi, La,
Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge and Nb. Hexagonal ferrite
powders containing the following elements can be generally used,
e.g., Co--Ti, Co--Ti--Zr, Co--Nb, Co--Ti--Zn, Co--Zn--Nb,
Ni--Ti--Zn, Nb--Zn, Ni--Ti, Zn--Ti and Zn--Ni. From the viewpoint
of SFD, pure M-type ferrites are preferred to composite type
ferrites full of spinel phase. Coercive force is controlled by the
methods of controlling the composition, tabular diameter and
tabular thickness of hexagonal ferrite, controlling the thickness
of a spinel phase, controlling the amount of the substitution
element of a spinel phase, and controlling the position of the
substitution site of a spinel phase.
[0051] Hexagonal ferrite magnetic powders for use in the invention
have an average tabular diameter of from 15 to 35 nm; a variation
coefficient of tabular diameters of from 0 to 30%; an average
tabular thickness of generally from 2 to 15 nm, particularly
preferably from 4 to 10 nm; and an average tabular ratio of
preferably from 1.5 to 4.5, more preferably from 2 to 4.2. When the
average tabular diameter of hexagonal ferrite magnetic powders is
in the above range, the specific surface area becomes an
appropriate value, so that hexagonal ferrite powders can be easily
dispersed. Hexagonal ferrite magnetic powders have a specific
surface area (S.sub.BET) of from 40 to 100 m.sup.2/g, more
preferably from 45 to 90 m.sup.2/g. When the specific surface area
is in this range, noise lowers and hexagonal ferrite powders can be
easily dispersed, so that surface property is improved. Hexagonal
ferrite magnetic powders have a moisture content of preferably from
0.3 to 2.0%. It is preferred to optimize the moisture content of
magnetic powders by the kind of a binder. The pH of hexagonal
ferrite magnetic powders is preferably optimized by the combination
with a binder to be used. The pH is from 5.0 to 12, and preferably
from 5.5 to 10.
[0052] Ferromagnetic powders may be subjected to treatment in
advance before dispersion with the later-described dispersant,
lubricant, surfactant and antistatic agent.
[0053] SFD of ferromagnetic powders themselves is preferably small,
and it is necessary to make the distribution of Hc of ferromagnetic
powders small. When SFD of a tape is small, magnetic flux
revolution is sharp and peak shift becomes small, so that the tape
is suitable for high density digital magnetic recording. For
achieving small Hc distribution, making the particle size
distribution of goethite in ferromagnetic metal powders good, using
monodispersed .alpha.-Fe.sub.2O.sub.3, and preventing sintering
among particles are effective methods.
[0054] Lower Layer:
[0055] A lower layer is described in detail below. A lower layer
preferably comprises nonmagnetic inorganic powder and a binder as
main components. Nonmagnetic inorganic powder for use in a lower
layer can be selected from inorganic compounds, e.g., metallic
oxide, metallic carbonate, metallic sulfate, metallic nitride,
metallic carbide and metallic sulfide. The examples of inorganic
compounds are selected from the following compounds and they can be
used alone or in combination, e.g., .alpha.-alumina having an
.alpha.-conversion rate of 90% or more, .beta.-alumina,
.gamma.-alumina, .theta.-alumina, silicon carbide, chromium oxide,
cerium oxide, .alpha.-iron oxide, hematite, goethite, corundum,
silicon nitride, titanium carbide, titanium oxide, silicon dioxide,
tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron
nitride, zinc oxide, calcium carbonate, calcium sulfate, barium
sulfate, and molybdenum disulfide. Of these compounds, titanium
dioxide, zinc oxide, iron oxide and barium sulfate are particularly
preferred for their small particle size distribution and various
means for imparting functions, and titanium dioxide and
.alpha.-iron oxide are more preferred. These nonmagnetic inorganic
powders preferably have an average particle size of from 0.005 to 2
.mu.m. A plurality of nonmagnetic inorganic powders each having a
different average particle size may be combined, if necessary, or a
single nonmagnetic inorganic powder having a broad particle size
distribution may be used so as to attain the same effect as such a
combination. A particularly preferred average particle size of
nonmagnetic inorganic powders is from 0.01 to 0.2 .mu.m. In
particular, when nonmagnetic inorganic powders are granular
metallic oxides, the average particle size of the granular metallic
oxides is preferably 0.08 .mu.m or less, and when nonmagnetic
inorganic powders are acicular metallic oxides, the average long
axis length of the acicular metallic oxides is preferably 0.3 .mu.m
or less, and more preferably 0.2 .mu.m or less. Nonmagnetic
inorganic powders for use in the invention have a tap density of
generally from 0.05 to2 g/ml, and preferably from 0.2 to 1.5 g/ml;
a moisture content of generally from 0.1 to 5 mass % (weight %),
preferably from 0.2 to 3 mass %, and more preferably from 0.3 to
1.5 mass %; a pH value of generally from 2 to 11, and particularly
preferably from 5.5 to 10; and a specific surface area of generally
from 1 to 100 m.sup.2/g, preferably from 5 to 80 m.sup.2/g, and
more preferably from 10 to 70 m.sup.2/g.
[0056] Nonmagnetic inorganic powders have a crystallite size of
preferably from 0.004 to 1 .mu.m, and more preferably from 0.04 to
0.1 .mu.m; an oil absorption amount using DBP (dibutyl phthalate)
of generally from 5 to 100 ml/100 g, preferably from 10 to 80
ml/100 g, and more preferably from 20 to 60 ml/100 g; and a
specific gravity of generally from 1 to 12, and preferably from 3
to 6. The configuration of nonmagnetic inorganic powders may be any
of acicular, spherical, polyhedral and tabular configurations.
Nonmagnetic inorganic powders have a Mohs' hardness of preferably
from 4 to 10; an adsorption amount of SA (stearic acid) of from 1
to 20 .mu.mol/m.sup.2, preferably from 2 to 15 .mu.mol/m.sup.2, and
more preferably from 3 to 8 .mu.mol/m.sup.2; and pH of from 3 to 6.
The surfaces of these nonmagnetic inorganic powders may be covered
with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, SnO.sub.2,
Sb.sub.2O.sub.3, ZnO or Y.sub.2O.sub.3. Al.sub.2O.sub.3, SiO.sub.2,
TiO.sub.2 and ZrO.sub.2 are preferred in the point of
dispersibility, and Al.sub.2O.sub.3, SiO.sub.2 and ZrO.sub.2 are
more preferred. These surface-covering compounds may be used in
combination or they may be used alone. Surface treatment may be
performed by coprecipitation, alternatively surface treatment of
powders may be performed in such a manner that particle surfaces
are covered with alumina in the first place, and then the
alumina-covered particles are covered with silica, or vice versa,
according to purposes. A surface-covered layer may be porous, if
necessary, but a homogeneous and dense layer is generally
preferred.
[0057] The specific examples and manufacturing methods of the
nonmagnetic inorganic powders for use in a lower layer in the
invention are disclosed in WO 98/35345.
[0058] By incorporating carbon blacks into a lower layer, surface
electrical resistance (Rs) and light transmittance can be reduced,
which are well known effects, and a desired micro Vickers hardness
can be obtained. It is also possible to bring about the effect of
stocking a lubricant by incorporating carbon blacks into a lower
layer. Furnace blacks for rubbers, thermal blacks for rubbers,
carbon blacks for coloring and acetylene blacks can be used as
carbon blacks. Carbon blacks used in a lower layer should optimize
the characteristics as described below according to the desired
effects and sometimes more effects can be obtained by the combined
use.
[0059] Carbon blacks for use in a lower layer have a specific
surface area of generally from 100 to 500 m.sup.2/g, preferably
from 150 to 400 m.sup.2/g, a DBP oil absorption amount of generally
from 20 to 400 ml/100 g, preferably from 30 to 400 ml/100 g, and an
average particle size of generally from 5to 80 nm, preferably from
10 to 50 nm, and more preferably from 10 to 40 nm. A small amount
of carbon blacks having an average particle size of 80 nm or
greater may be contained. Carbon blacks preferably have pH of from
2 to 10, a moisture content of from 0.1 to 10%, and a tap density
of from 0.1 to 1 g/ml.
[0060] The specific examples of carbon blacks for use in a lower
layer are disclosed in WO 98/35345. Carbon blacks can be used in
the range not exceeding 50 mass % based on the above nonmagnetic
inorganic powders (not including carbon blacks) and not exceeding
40% based on the total mass of nonmagnetic layers. Carbon blacks
can be used alone or in combination. Regarding the carbon blacks
for use in the present invention, compiled by Carbon Black
Association, Carbon Black Binran (Handbook of Carbon Blacks) can be
referred to.
[0061] Organic powders can be used in a lower layer according to
purpose, e.g., acrylic styrene resin powders, benzoguanamine resin
powders, melamine resin powders and phthalocyanine pigments are
exemplified. In addition, polyolefin resin powders, polyester resin
powders, polyamide resin powders, polyimide resin powders and
polyethylene fluoride resin powders can also be used. The producing
methods of these organic powders are disclosed in JP-A-62-18564 and
JP-A-60-255827.
[0062] The binder resins, lubricants, dispersants, additives,
solvents, dispersing methods and others used in a magnetic layer
described later can be used in a lower layer. In particular, with
respect to the amounts and the kinds of binder resins, additives,
the amounts and the kinds of dispersants, well-known techniques
regarding a magnetic layer can be applied to a lower layer.
[0063] Binder:
[0064] Conventionally well-known thermoplastic resins,
thermosetting resins, reactive resins and the mixtures of these
resins are used as a binder in the invention.
[0065] Thermoplastic resins having a glass transition temperature
of from -100 to 150.degree. C., a number average molecular weight
of from 1,000 to 200,000, preferably from 10,000 to 100,000, and a
polymerization degree of from about 50 to about 1,000 can be used
in the invention.
[0066] The examples of these thermoplastic resins include polymers
or copolymers containing, as the constituting unit, vinyl chloride,
vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic
ester, vinylidene chloride, acrylonitrile, methacrylic acid,
methacrylic ester, styrene, butadiene, ethylene, vinyl butyral,
vinyl acetal or vinyl ether; polyurethane resins and various rubber
resins. The examples of thermosetting resins and reactive resins
include phenolic resins, epoxy resins, curable type polyurethane
resins, urea resins, melamine resins, alkyd resins, acrylic
reactive resins, formaldehyde resins, silicone resins,
epoxy-polyamide resins, mixtures of polyester resins and isocyanate
prepolymers, mixtures of polyesterpolyol and polyisocyanate, and
mixtures of polyurethane and polyisocyanate. These resins are
described in detail in Plastic Handbook, Asakura Shoten. It is also
possible to use well-known electron beam-curable type resins in
each layer. The examples of these resins and manufacturing methods
are disclosed in detail in JP-A-62-256219. These resins can be used
alone or in combination. The examples of preferred combinations
include at least one resin selected from vinyl chloride resins,
vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl
acetate-vinyl alcohol copolymers, and vinyl chloride-vinyl
acetate-maleic anhydride copolymers with a polyurethane resin, and
combinations of these resins with polyisocyanate.
[0067] Polyurethane resins having well known structures, e.g.,
polyester polyurethane, polyether polyurethane, polyether polyester
polyurethane, polycarbonate polyurethane, polyester polycarbonate
polyurethane and polycaprolactone polyurethane can be used. With
respect to all the binders described above, for the purpose of
obtaining more excellent dispersibility and durability, it is
preferred to use at least one polar group selected from the
following by copolymerization or addition reaction, according to
necessity, e.g., --COOM, --SO.sub.3M, --OSO.sub.3M,
--P.dbd.O(OM).sub.2, --O--P.dbd.O(OM).sub.2 (wherein M represents a
hydrogen atom or an alkali metal salt group), --NR.sub.2,
--N.sup.+R.sub.3 (wherein R represents a hydrocarbon group), an
epoxy group, --SH and --CN. The amount of the polar group added is
from 10.sup.-1 to 10.sup.-8 mol/g, preferably from 10.sup.-2 to
10.sup.-6 mol/g. It is preferred for polyurethane resins to have at
least one OH group at each terminal of a polyurethane molecule,
i.e., two or more in total, besides the above polar groups. Since
OH groups form a three dimensional network structure by
crosslinking with a polyisocyanate curing agent, they are
preferably contained in a molecule as many as possible. In
particular, it is preferred that OH groups be present at terminals
of a molecule, since the reactivity with the curing agent becomes
high. It is preferred for polyurethane to have three or more OH
groups, particularly preferably four or more OH groups, at
terminals of a molecule. When polyurethane is used in the
invention, the polyurethane has a glass transition temperature of
generally from -50 to 150.degree. C., preferably from 0 to
100.degree. C., and particularly preferably from 30 to 100.degree.
C., breaking extension of from 100 to 2,000%, breaking stress of
generally from 0.05 to 10 kg/mm.sup.2 (=about 0.49 to 98 MPa), and
a yielding point of from 0.05 to 10 kg/mm.sup.2 (=about 0.49 to 98
MPa). Due to these physical properties, a coating film having good
mechanical properties can be obtained.
[0068] The specific examples of binders for use in the invention
include as vinyl chloride copolymers VAGH, VYHH, VMCH, VAGF, VAGD,
VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC and PKFE
(manufactured by Union Carbide Co., Ltd.), MPR-TA, MPR-TAS,
MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM and MPR-TAO (manufactured
by Nisshin Chemical Industry Co., Ltd.), 1000W, DX80, DX81, DX82,
DX83 and 100FD (manufactured by Electro Chemical Industry Co.,
Ltd.), MR-104, MR-105, MR-110, MR-100, MR-555 and 400X-110A
(manufactured by Nippon Zeon Co., Ltd.); and as polyurethane resins
Nippollan N2301, N2302 and N2304 (manufactured by Nippon
Polyurethane Co., Ltd.), Pandex T-5105, T-R3080, T-5201, Burnock
D-400, D-210-80, Crisvon 6109 and 7209 (manufactured by Dainippon
Ink and Chemicals Inc.), Vylon UR8200, UR8300, UR8700, RV530 and
RV280 (manufactured by Toyobo Co., Ltd.), polycarbonate
polyurethane, Daipheramine 4020, 5020, 5100, 5300, 9020, 9022 and
7020 (manufactured by Dainichiseika Color & Chemicals Mfg. Co.,
Ltd), polyurethane, MX5004 (manufactured by Mitsubishi Kasei
Corp.), polyurethane, Sanprene SP-150 (manufactured by Sanyo
Chemical Industries, Co., Ltd.), and polyurethane, Saran F310 and
F210 (manufactured by Asahi Kasei Corporation).
[0069] The amount of binders for use in a nonmagnetic layer and a
magnetic layer is from 5 to 50 mass %, preferably from 10 to 30
mass %, respectively based on the nonmagnetic inorganic powder and
the magnetic powder. When vinyl chloride resins are used, the
amount is from 5 to 30 mass %, when polyurethane resins are used,
the amount is from 2 to 20 mass %, and it is preferred to use
polyisocyanate in an amount of from 2 to 20 mass % in combination
with these resins, however, for instance, when the corrosion of
heads is caused by a slight amount of chlorine due to
dechlorination, it is also possible to use polyurethane alone or a
combination of polyurethane and isocyanate alone.
[0070] A magnetic recording medium according to the invention
comprises two or more layers, the amount of binder, the amounts of
vinyl chloride resin, polyurethane resin, polyisocyanate or other
resins contained in a binder, the molecular weight of each resin
constituting a magnetic layer, the amount of polar groups, or the
physical properties of the above-described resins can of course be
varied in each layer according to necessity. These factors should
be rather optimized in each layer. Well-known techniques with
respect to multilayer magnetic layers can be used in the invention.
For example, when the amount of a binder is varied in each layer,
it is effective to increase the amount of a binder contained in a
magnetic layer to reduce scratches on the magnetic layer surface.
For improving the head touch against a head, it is effective to
increase the amount of the binder in a nonmagnetic layer to impart
flexibility.
[0071] The examples of polyisocyanates for use in the invention
include isocyanates, e.g., tolylenediisocyanate,
4,4'-diphenylmethanediisocyanate- , hexamethylenediisocyanate,
xylylenediisocyanate, naphthylene-1,5-diisocy- anate,
o-toluidinediisocyanate, isophoronediisocyanate and
triphenyl-methanetriisocyanate; products of these isocyanates with
polyalcohols; and polyisocyanates formed by condensation reaction
of isocyanates. These isocyanates are commercially available under
the trade names of Coronate L, Coronate HL, Coronate 2030, Coronate
2031, Millionate MR and Millionate MTL (manufactured by Nippon
Polyurethane Co., Ltd.), Takenate D-102, Takenate D-110N, Takenate
D-200 and Takenate D-202 (manufactured by Takeda Chemical
Industries, Ltd.), and Desmodur L, Desmodur IL, Desmodur N and
Desmodur HL (manufactured by Sumitomo Bayer Co., Ltd.). These
compounds may be used alone, or in combination of two or more in
each layer taking advantage of the difference in curing
reactivity.
[0072] Carbon Black, Abrasive:
[0073] Carbon blacks for use in a magnetic layer in the present
invention include furnace blacks for rubbers, thermal blacks for
rubbers, carbon blacks for coloring, and acetylene blacks. Carbon
blacks for use in the present invention have a specific surface
area of from 5 to 500 m.sup.2/g, a DBP oil absorption amount of
from 10 to 400 ml/100 g, an average particle size of from 5 to 300
nm, a pH value of from 2 to 10, a moisture content of from 0.1 to
10%, and a tap density of from 0.1 to 1 g/ml. The specific examples
of these carbon blacks are disclosed in WO 98/35345.
[0074] Carbon blacks can serve various functions such as the
prevention of static charges of a magnetic layer, the reduction of
a friction coefficient, the impartation of a light-shielding
property and the improvement of film strength. Such functions
differ depending upon the kind of carbon blacks to be used.
Accordingly, when the invention takes a multilayer structure, it is
of course possible to select and determine the kinds, the amounts
and the combinations of the carbon blacks to be added to each layer
on the basis of the above-described various properties such as the
particle size, the oil absorption amount, the electrical
conductance and the pH value, or these should be rather optimized
in each layer.
[0075] It is preferred to use diamond particles as an abrasive in
the invention. Diamond particles have an average particle size of
preferably from 1/5 to 2 times the thickness of a magnetic layer,
more preferably 1/2 to 1.5 times, still more preferably from 0.8 to
1.2 times. The blending ratio of diamond particles is preferably
from 0.1 to 5.0 mass % of the ferromagnetic powders, and more
preferably from 0.5 to 3 mass %.
[0076] Abrasives other than diamond particles can be used in
combination in a magnetic layer in the present invention. Well
known materials essentially having a Mohs' hardness of 6 or higher
can be used as abrasives in a magnetic layer in the invention alone
or in combination, e.g., .alpha.-alumina having an
.alpha.-conversion rate of 90% or more, .beta.-alumina, silicon
carbide, chromium oxide, cerium oxide, .alpha.-iron oxide,
corundum, silicon nitride, titanium carbide, titanium oxide,
silicon dioxide and boron nitride. The composites of these
abrasives (abrasives obtained by surface-treating with other
abrasives) may also be used. Compounds or elements other than their
main components are often contained in abrasives, but the intended
effects can be attained so long as the content of main component is
90% or more. These abrasives preferably have an average particle
size of from 0.01 to 2 .mu.m and, in particular, for improving
electromagnetic characteristics, it is preferred to use abrasives
having narrow particle size distribution. For improving durability,
abrasives each having different particle size may be combined
according to necessity, or a single abrasive having broad particle
size distribution may be used so as to attain the same effect as
such a combination. Abrasives for use in the invention preferably
have a tap density of from 0.3 to 2 g/ml, a moisture content of
from 0.1 to 5%, a pH value of from 2 to 11, and a specific surface
area of from 1 to 30 m.sup.2/g. The configurations of abrasives for
use in the invention may be any of acicular, spherical and die-like
configurations, but abrasives having a configuration partly with
edges are preferred for their high abrasive property. The specific
examples of these abrasives are disclosed in WO 98/35345. The
particle sizes and the amounts of abrasives to be added to a
magnetic layer and a nonmagnetic layer should be independently set
at optimal values.
[0077] Additive:
[0078] As additives for use in a magnetic layer and a nonmagnetic
layer in the invention, those having a lubricating effect, an
antistatic effect, a dispersing effect and a plasticizing effect
are used and comprehensive improvement of performances can be
contrived by combining additives. As additives having a lubricating
effect, lubricants giving a remarkable action on agglutination
caused by the friction of surfaces of materials with each other are
used. Lubricants are classified into two types. Lubricants that are
used for a magnetic disc cannot be judged completely whether they
show fluid lubrication or boundary lubrication, but according to
general concept they are classified into higher fatty acid esters,
liquid paraffin and silicon derivatives which show fluid
lubrication, and long chain fatty acids, fluorine surfactants and
fluorine-containing polymers which show boundary lubrication. In a
coating type magnetic recording medium, lubricants exist in a state
dissolved in a binder or in a state of partly being adsorbed onto
the surface of hexagonal ferrite magnetic powder, and they migrate
to the surface of a magnetic layer. The speed of migration depends
upon whether the compatibility of a binder and a lubricant is good
or bad. The speed of migration is slow when the compatibility of a
binder and a lubricant is good and the migration speed is fast when
the compatibility is bad. As one idea as to good or bad of the
compatibility, there is a means of comparison of dissolution
parameters of a binder and a lubricant. A nonpolar lubricant is
effective for fluid lubrication and a polar lubricant is effective
for boundary lubrication.
[0079] In the present invention, it is preferred to use a higher
fatty acid ester showing fluid lubrication and a long chain fatty
acid showing boundary lubrication each having different
characteristics in combination, and it is more preferred to combine
at least three of these lubricants. Solid lubricants can also be
used in combination with these lubricants.
[0080] The examples of solid lubricants that can be used in
combination include molybdenum disulfide, tungsten disulfide,
graphite, boron nitride and graphite fluoride. The examples of long
chain fatty acids showing boundary lubrication include monobasic
fatty acids having from 10 to 24 carbon atoms (they may contain an
unsaturated bond or may be branched) and metal salts of these
monobasic fatty acids (e.g., with Li, Na, K or Cu). The examples of
fluorine surfactants and fluorine-containing polymers include
fluorine-containing silicones, fluorine-containing alcohols,
fluorine-containing esters, fluorine-containing alkyl sulfates and
alkali metal salts of these compounds. The examples of higher fatty
acid esters showing fluid lubrication include fatty acid
monoesters, fatty acid diesters and fatty acid triesters composed
of a monobasic fatty acid having from 10 to 24 carbon atoms (which
may contain an unsaturated bond or may be branched) and any one of
mono-, di-, tri-, tetra-, penta- and hexa-alcohols having from 2 to
12 carbon atoms (which may contain an unsaturated bond or may be
branched), and fatty acid esters of monoalkyl ethers of alkylene
oxide polymers. In addition to the above, the examples further
include liquid paraffin, and as silicon derivatives, silicone oils,
e.g., dialkylpolysiloxane (the alkyl group has from 1 to 5 carbon
atoms), dialkoxypolysiloxane (the alkoxyl group has from 1 to 4
carbon atoms), monoalkyl-monoalkoxypolysiloxane (the alkyl group
has from 1 to 5 carbon atoms and the alkoxyl group has from 1 to 4
carbon atoms), phenylpolysiloxane, and fluoroalkylpolysiloxane (the
alkyl group has from 1 to 5 carbon atoms), silicones having a polar
group, fatty acid-modified silicones, and fluorine-containing
silicones.
[0081] The examples of other lubricants include alcohols, e.g.,
mono-, di-, tri-, tetra-, penta- and hexa-alcohols having from 12
to 22 carbon atoms (they may contain an unsaturated bond or may be
branched), alkoxy alcohols having from 12 to 22 carbon atoms (they
may contain an unsaturated bond or may be branched), and
fluorine-containing alcohols, polyethylene waxes, polyolefins such
as polypropylene, ethylene glycols, polyglycols such as
polyethylene oxide waxes, alkyl phosphates and alkali metal salts
of alkyl phosphates, alkyl sulfates and alkali metal salts of alkyl
sulfates, polyphenyl ethers, fatty acid amides having from 8 to 22
carbon atoms, and aliphatic amines having from 8 to 22 carbon
atoms.
[0082] The examples of additives having an antistatic effect, a
dispersing effect and a plasticizing effect include
phenylphosphonic acid, specifically "PPA" (manufactured by Nissan
Chemical Industries, Ltd.), .alpha.-naphthylphosphoric acid,
phenylphosphoric acid, diphenylphosphoric acid,
p-ethyl-benzenephosphonic acid, phenylphosphinic acid,
aminoquinones, various kinds of silane coupling agents, titanium
coupling agents, fluorine-containing alkyl sulfates and alkali
metal salts of these compounds.
[0083] Lubricants that are particularly preferably used in the
invention are fatty acids and fatty acid esters, and the specific
examples are disclosed in WO98/35345. Besides the above, other
different lubricants and additives can be used in combination as
well.
[0084] Additionally, nonionic surfactants, e.g., alkylene oxides,
glycerols, glycidols and alkylphenol-ethylene oxide adducts;
cationic surfactants, e.g., cyclic amines, ester amides, quaternary
ammonium salts, hydantoin derivatives, heterocyclic rings,
phosphoniums and sulfoniums; anionic surfactants containing an acid
group, such as carboxylic acid, sulfonic acid, phosphoric acid, a
sulfuric ester group and a phosphoric ester group; and amphoteric
surfactants, e.g., amino acids, aminosulfonic acids, sulfuric
esters or phosphoric esters of amino alcohols, and alkylbetaines
can also be used. These surfactants are described in detail in
Kaimen Kasseizai Binran (Handbook of Surfactants) (published by
Sangyo Tosho Co., Ltd.). These lubricants and antistatic agents
need not be 100% pure and may contain impurities such as isomers,
unreacted products, byproducts, decomposed products and oxides, in
addition to the main component. However, the content of impurities
is preferably 30% or less, more preferably 10% or less.
[0085] As disclosed in WO 98/35345, it is also preferred to use a
monoester and a diester in combination as fatty acid esters in the
present invention.
[0086] The surface of a magnetic layer in the invention has a C/Fe
peak ratio measured by Auger electron spectroscopy of preferably
from 5 to 100, particularly preferably from 5 to 80. The measuring
conditions of the C/Fe peak ratio by Auger electron spectroscopy
are as follows.
[0087] Instrument: Model PHI-660, manufactured by .PHI. Co.
[0088] Measuring Conditions:
[0089] Primary electron beam accelerating voltage: 3 KV
[0090] Electric current of sample: 130 nA
[0091] Magnification: 250-fold
[0092] Inclination angle: 30.degree.
[0093] The value of C/Fe peak ratiois obtained as the C/Fe ratio by
integrating the values obtained under the above conditions in the
region of kinetic energy of 130 eV to 730 eV three times and
finding the strengths of KILL peak of the carbon and LMM peak of
the iron as differentials.
[0094] The amount of the lubricants contained in each of an upper
layer and a lower layer of a magnetic recording medium in the
invention is preferably from 5 to 30 mass parts per 100 mass parts
of the ferromagnetic powder and the nonmagnetic inorganic powder
respectively.
[0095] Lubricants and surfactants for use in the invention
individually have different physical functions. The kinds, amounts
and combining proportions bringing about synergistic effects of
these lubricants should be determined optimally in accordance with
the purpose. A nonmagnetic layer and a magnetic layer can
separately contain different fatty acids each having a different
melting point so as to prevent bleeding out of the fatty acids to
the surface, or different esters each having a different boiling
point, a different melting point or a different polarity so as to
prevent bleeding out of the esters to the surface. Also, the amount
of the surfactant is controlled so as to improve the coating
stability, or the amount of the lubricant in the intermediate layer
is made larger so as to improve the lubricating effect. The
examples are by no means limited thereto. In general, the total
amount of lubricants is from 0.1 to 50 mass %, preferably from 2 to
25 mass %, based on the amount of the ferromagnetic powder or the
nonmagnetic powder.
[0096] All or a part of the additives to be used in the invention
may be added to a magnetic coating solution or a nonmagnetic
coating solution in any step of preparation. For example, additives
may be blended with magnetic powder before a kneading step, may be
added in a step of kneading magnetic powder, a binder and a
solvent, may be added in a dispersing step, may be added after a
dispersing step, or may be added just before coating. According to
the purpose, there are cases of capable of attaining the object by
coating all or a part of additives simultaneously with or
successively after the coating of a magnetic layer. Further,
according to purpose, a lubricant maybe coated on the surface of a
magnetic layer after calendering treatment or after completion of
slitting.
[0097] Layer Constitution:
[0098] The thickness of the support of a magnetic disc in the
invention is generally from 2 to 100 .mu.m, preferably from 2 to 80
.mu.m.
[0099] An undercoat layer may be provided between a support,
preferably a nonmagnetic flexible support, and a nonmagnetic or
magnetic layer for adhesion improvement. The thickness of the
undercoat layer is from 0.01 to 0.5 .mu.m, preferably from 0.02 to
0.5 .mu.m.
[0100] A backing layer may be provided on the side of a support
opposite to the side having a magnetic layer for the purpose of
providing static charge prevention and curling correction. The
thickness of the backing layer is generally from 0.1 to 4 .mu.m,
preferably from 0.3 to 2.0 .mu.m. Well-known undercoat layers and
backing layers can be used for this purpose.
[0101] The thickness of a magnetic layer having the constitution
comprising a lower layer and an upper layer in the invention is as
described above, but the thickness is optimized by the amount of
saturation magnetization of the head to be used, the head gap
length and the recording signal zone. The thickness of a lower
layer is generally from 0.2 to 5.0 .mu.m, preferably from 0.3 to
3.0 .mu.m, and more preferably from 1.0 to 2.5 .mu.m.
[0102] A lower layer exhibits the effect of the invention so long
as it is substantially nonmagnetic even if, or intentionally, it
contains a small amount of magnetic powder as the impurity, which
can be as a matter of course regarded as essentially the same
constitution as in the invention. The terminology "substantially
nonmagnetic" means that the residual magnetic flux density of a
lower layer is 100 mT or less or the coercive force of a lower
layer is 100 Oe (=about 8 kA/m) or less, preferably the residual
magnetic flux density and the coercive force are zero. When a lower
layer contains magnetic powder, the content of the magnetic powder
is preferably less than 1/2 of the total inorganic powders
contained in the lower layer. In place of a nonmagnetic layer, a
soft magnetic layer containing soft magnetic powder and a binder
may be formed as a lower layer. The thickness of the soft magnetic
layer is the same as the thickness of a lower layer as described
above.
[0103] Support:
[0104] A support for use in the invention is preferably a
nonmagnetic flexible support, and essentially has a thermal
shrinkage factor of preferably 0.5% or less at 100.degree. C. for
30 minutes, and of preferably 0.5% or less at 80.degree. C. for 30
minutes, more preferably 0.2% or less, in every direction of
in-plane of the support. Further, the thermal shrinkage factors of
the support at 100.degree. C. for 30 minutes and at 80.degree. C.
for 30 minutes are preferably almost equal in every direction of
in-plane of the support with difference of not more than 10%. The
support is preferably a nonmagnetic support. As nonmagnetic
supports, well-known films such as polyesters (e.g., polyethylene
terephthalate and polyethylene naphthalate), polyolefins, cellulose
triacetate, polycarbonate, aromatic or aliphatic polyamide,
polyimide, polyamideimide, polysulfone and polybenzoxazole can be
used. High strong supports such as polyethylene naphthalate and
polyamide are preferably used. If necessary, a lamination type
support as disclosed in JP-A-3-224127 can be used to vary the
surface roughness of a magnetic layer surface and a base surface.
These supports may be subjected in advance to surface activation
treatment, e.g., corona discharge treatment, plasma treatment,
adhesion assisting treatment, heat treatment or dust-removing
treatment. Aluminum or glass substrate can also be used as a
support in the invention.
[0105] For attaining the object of the invention, it is preferred
to use a support having a central plane average surface roughness
(Ra) of 4.0 nm or less, preferably 2.0 nm or less, measured by a
surface roughness meter TOPO-3D (a product of WYKO Co.). It is
preferred that the support not only has a small central plane
average surface roughness but also is free from coarse spines
having heights of 0.5 .mu.m or more. Surface roughness
configuration is freely controlled by the size and the amount of a
filler added to a support. The examples of fillers include oxides
and carbonates of Ca, Si and Ti, and acrylic-based organic powders.
A support for use in the invention preferably has a maximum height
(Rmax) of 1 .mu.m or less, a ten point average roughness (Rz) of
0.5 .mu.m or less, a central plane peak height (Rp) of 0.5 .mu.m or
less, a central plane valley depth (Rv) of 0.5 .mu.m or less, a
central plane area factor (Sr) of from 10% to 90%, and average
wavelength (.lambda.a) of from 5 to 300 .mu.m. For obtaining
desired electromagnetic characteristics and durability, the spine
distribution on the surface of a support can be controlled
arbitrarily by using fillers, e.g., the number of spines having
sizes of from 0.01 to 1 .mu.m can be controlled each within the
range of from 0 to 2,000 per 0.1 mm.sup.2.
[0106] Supports for use in the invention have an F-5 value of
preferably from 5 to 50 kg/mm.sup.2 (=about 49 to 490 MPa), a
thermal shrinkage factor at 100.degree. C. for 30 minutes of
preferably 3% or less, more preferably 1.5% or less, a thermal
shrinkage factor at 80.degree. C. for 30 minutes of preferably 1%
or less, more preferably 0.5% or less, a breaking strength of from
5 to 100 kg/mm.sup.2 (=about 49 to 980 MPa), an elastic modulus of
from 100 to 2,000 kg/mm.sup.2 (=about 0.98 to 19.6 GPa), a
temperature expansion coefficient of from 10.sup.-4 to
10.sup.-8/.degree. C., preferably from 10.sup.-5 to
10.sup.-6/.degree. C., and a humidity expansion coefficient of
10.sup.-4/RH % or less, preferably 10.sup.-5/RH % or less. These
thermal, dimensional and mechanical strength characteristics are
preferably almost equal in every direction of in-plane of supports
with difference of not more than 10%.
[0107] Manufacturing Method:
[0108] The manufacturing process of a magnetic coating solution of
a magnetic disc in the invention comprises at least a kneading
step, a dispersing step and optionally a blending step to be
carried out before and/or after the kneading and dispersing steps.
Each of these steps may be composed of two or more separate stages.
All the feedstock such as magnetic powder, nonmagnetic powder, a
binder, a carbon black, an abrasive, an antistatic agent, a
lubricant and a solvent for use in the invention may be added at
any step at any time. Each feedstock may be added at two or more
steps dividedly. For example, polyurethane can be added dividedly
at a kneading step, a dispersing step, or a blending step for
adjusting viscosity after dispersion. For achieving the object of
the invention, conventionally well-known techniques can be
performed partly with the above steps. Powerful kneading machines
such as an open kneader, a continuous kneader, a pressure kneader
and an extruder are preferably used in a kneading step. When a
kneader is used, all or a part of the binder (preferably 30% or
more of the total binder) is kneaded in the range of from 15 to 500
parts per 100 parts of the magnetic powder together with the
magnetic powder or nonmagnetic powder. These kneading treatments
are disclosed in detail in JP-A-1-106338 and JP-A-1-79274. For
dispersing a magnetic layer coating solution and a nonmagnetic
layer coating solution, glass beads can be used, but dispersing
media having a high specific gravity, e.g., zirconia beads, titania
beads and steel beads are preferred for this purpose. Optimal
particle size and packing density of these dispersing media have to
be selected. Well-known dispersers can be used in the
invention.
[0109] After coating a coating solution on a support, the magnetic
disc is subjected to orientation treatment as desired.
[0110] In the case of a magnetic disc, an isotropic orienting
property can be sufficiently obtained in some cases without
performing orientation with orientating apparatus, but it is
preferred to use well-known random orientation apparatus, e.g.,
disposing cobalt magnets diagonally and alternately or applying an
alternating current magnetic field with a solenoid. Hexagonal
ferrite magnetic powders have generally an inclination for
three-dimensional random orientation of in-plane and in the
perpendicular direction, however, it is also possible to make
in-plane two-dimensional random orientation. It is also possible to
impart isotropic magnetic characteristics in the circumferential
direction by perpendicular orientation using well-known methods,
e.g., using different pole and counter position magnets. In
particular, perpendicular orientation is preferred when the disc is
used in high density recording. Circumferential orientation can be
performed using spin coating.
[0111] After coating and drying, the web having the coated layer is
preferably subjected to calendering treatment.
[0112] Heat resistive plastic rolls, e.g., epoxy, polyimide,
polyamide and polyimideamide or metal rolls are used as calendering
treatment rolls. Metal rolls are preferably used for the treatment
particularly when magnetic layers are coated on both surfaces of a
support. The treatment temperature is preferably 50.degree. C. or
more, more preferably 100.degree. C. or more. The linear pressure
is preferably 200 kg/cm (=about 196 kN/m) or more, more preferably
300 kg/cm (=about 294 kN/m) or more.
[0113] Physical Properties:
[0114] Residual magnetic flux density.times.magnetic layer
thickness of a magnetic disc in the invention is preferably from 5
to 300 mT.multidot..mu.m. The coercive force (Hc) is preferably
from 1,800 to 5,000 Oe (=about 144 to 400 kA/m), more preferably
from 1,800 to 3,000 Oe (=about 144 to 240 kA/m). The distribution
of the coercive force is preferably narrow, and SFD (switching
field distribution) and SFDr are preferably 0.6 or less.
[0115] The squareness ratio (SQ) of a magnetic disc is from 0.55 to
0.67, preferably from 0.58 to 0.64, in the case of two dimensional
random orientation, from 0.45 to 0.55 in the case of three
dimensional random orientation, and in the case of perpendicular
orientation generally 0.6 or more in the perpendicular direction,
preferably 0.7 or more, and 0.7 or more when diamagnetic correction
is performed, preferably 0.8 or more. Degree of orientation in
two-dimensional random orientation and three-dimensional random
orientation is preferably 0.8 or more. In the case of
two-dimensional random orientation, the squareness ratio in the
perpendicular direction, the Br in the perpendicular direction, and
the Hc in the perpendicular direction are preferably from 0.1 to
0.5 times as small as those in the in-plane direction.
[0116] A magnetic recording medium in the invention has the
intrinsic resistivity of the surface of a magnetic layer of
preferably from 10.sup.4 to 10.sup.12 .OMEGA./sq, and a charge
potential of preferably from -500 V to +500 V. The elastic modulus
at 0.5% elongation of a magnetic layer is preferably from 100 to
2,000 kg/mm2 (=about 980 to 19,600 MPa) in every direction of
in-plane, the breaking strength is preferably from 10 to 70
kg/mm.sup.2 (=about 98 to 686 MPa), the elastic modulus of a
magnetic disc is preferably from 100 to 1,500 kg/mm.sup.2 (=about
980 to 14,700 MPa) in every direction of in-plane, the residual
elongation is preferably 0.5% or less, and the thermal shrinkage
factor at every temperature of 100.degree. C. or less is preferably
1% or less, more preferably 0.5% or less, and most preferably 0.1%
or less. The glass transition temperature of a magnetic layer (the
maxium point of the loss elastic modulus by dynamic viscoelasticity
measurement at 110 Hz) is preferably from 50 to 120.degree. C., and
that of a lower layer is preferably from 0 to 100.degree. C. The
loss elastic modulus is preferably in the range of from
1.times.10.sup.7 to 8.times.10.sup.8 Pa, and loss tangent is
preferably 0.2 or less. When loss tangent is too large, adhesion
failure is liable to occur. These thermal and mechanical
characteristics are preferably almost equal in every direction of
in-plane of the medium with difference of not more than 10%. The
residual amount of a solvent in a magnetic layer is preferably 100
mg/m.sup.2 or less, more preferably 10 mg/m.sup.2 or less. The void
ratio of a coated layer is preferably 30% by volume or less, more
preferably 20% by volume or less, with both of a lower layer and an
upper layer. The void ratio is preferably smaller for obtaining
high output but in some cases a specific value should be preferably
secured depending upon purposes. For example, in a disc medium that
is repeatedly used, large void ratio contributes to good running
durability in many cases.
[0117] A magnetic layer surface has a central plane average surface
roughness (Ra) measured with a surface roughness meter TOPO-3D (a
product of WYKO Co.) of preferably 5.0 nm or less, more preferably
4.0 nm or less, and especially preferably 3.5 nm or less. A
magnetic layer preferably has a maximum height (Rmax) of 0.5 .mu.m
or less, a ten point average roughness (Rz) of 0.3 .mu.m or less, a
central plane peak height (Rp) of 0.3 .mu.m or less, a central
plane valley depth (Rv) of 0.3 .mu.m or less, a central plane area
factor (Sr) of from 20% to 80%, and average wavelength (.lambda.a)
of from 5 to 300 .mu.m. The surface spines of a magnetic layer of
sizes of from 0.01 to 1 .mu.m can be controlled arbitrarily within
the range of from 0 to 2,000, and surface spines are preferably
optimized. Surface spines can be easily controlled by the control
of the surface property of a support by using fillers, the particle
size and the amount of magnetic powders added to a magnetic layer,
or by the surface configurations of the rolls of calender
treatment. Curing is preferably within .+-.3 mm. It can be easily
presumed that these physical properties of a magnetic disc in the
invention can be varied according to purposes in a lower layer and
an upper layer. For example, the elastic modulus of an upper layer
is made higher to improve running durability and at the same time
the elastic modulus of a lower layer is made lower that that of the
upper layer to improve the head touching of the magnetic disc.
EXAMPLES
[0118] The invention is specifically described with reference to
examples and comparative examples. "Parts" in the following means
"parts by mass".
[0119] Sample 1:
[0120] Each composition of magnetic coating solution A and
nonmagnetic coating solution shown below was blended in a kneader,
and magnetic coating solution A was dispersed with a sand mill at
2,000 rpm for 12 hours, and the nonmagnetic coating solution was
dispersed with a sand mill at 2,000 rpm for 3 hours. Polyisocyanate
was added to each dispersion of the obtained magnetic coating
solution A and nonmagnetic coating solution, in an amount of 3
parts to magnetic coating solution A and 6 parts to the nonmagnetic
coating solution, and 30 parts of cyclohexanone was added to each
solution. Each solution was filtered through a filter having an
average pore diameter of 1 .mu.m to obtain coating solutions for
forming a magnetic layer and a nonmagnetic layer.
[0121] The thus-obtained nonmagnetic coating solution was coated on
a polyethylene naphthalate support having a thickness of 53 .mu.m
and a central plane average surface roughness of 3 nm in a dry
thickness of 1.5 .mu.m and dried, and then the magnetic coating
solution was coated in a thickness of 0.15 .mu.m. After drying, the
coated layer was subjected to calendering treatment with
seven-stage calender at 90.degree. C. and linear pressure of 300
kg/cm. The obtained web was punched out to a disc of 3.7 inches,
and the disc was further heat-treated in a thermostat at 55.degree.
C. for 24 hours.
[0122] Samples 2 and 3:
[0123] Samples 2 and 3 were produced in the same manner as in the
preparation of sample 1 except that the amount of isocetyl stearate
in magnetic coating solution A or the nonmagnetic coating solution
was changed to the amount shown in Table 1 below.
[0124] Sample 4:
[0125] Sample 4 was produced in the same manner as in the
preparation of sample 1 except that magnetic coating solution B
shown below was used in place of magnetic coating solution A, each
composition was blended in a kneader, dispersed with a sand mill at
2,000 rpm for 6 hours, 1 part of carbon black #50 (manufactured by
Asahi Carbon Co., Ltd.) was added to each coating solution, and
further dispersed for 6 hours.
[0126] Sample 5:
[0127] Sample 5 was produced in the same manner as in the
preparation of sample 1 except that magnetic coating solution B
shown below was used in place of magnetic coating solution A, each
composition was blended in a kneader, dispersed with a sand mill at
2,000 rpm for 8 hours, 1 part of carbon black #50 (manufactured by
Asahi Carbon Co., Ltd.) was added to each coating solution, and
further dispersed for 4 hours.
[0128] Samples 6 to 10:
[0129] Samples 6 to 10 were produced in the same manner as in the
preparation of sample 4 except that the amount of isocetyl stearate
in the magnetic coating solution or the nonmagnetic coating
solution was changed or the kind and amount of isocetyl stearate in
the magnetic coating solution and the nonmagnetic coating solution
were changed to the ester and the amount as shown in Table 1.
[0130] Samples 11 and 12:
[0131] Samples 11 and 12 were produced in the same manner as in the
preparation of sample 1 except that the amount of isocyanate added
to magnetic coating solution A or the nonmagnetic coating solution
was changed to the amount shown in Table 1.
[0132] Samples 13 to 15:
[0133] Samples 13 to 15 were produced in the same manner as in the
preparation of sample 4 except that the amount of isocyanate added
to the magnetic coating solution or the nonmagnetic coating
solution was changed to the amount shown in Table 1.
[0134] Samples 16 and 17:
[0135] Samples 16 and 17 were produced in the same manner as in the
preparation of sample 1 except that the average particle size of
diamond particles added to magnetic coating solution A was changed
to the average particle size as shown in Table 1.
[0136] Sample 18:
[0137] Sample 18 was produced in the same manner as in the
preparation of sample 1 except that that magnetic coating solution
C shown below was used in place of magnetic coating solution A,
each composition was blended in a kneader, dispersed with a
sandmill at 2,000 rpm for 12 hours, and a solution obtained by
slurrying polyisocyanate and 2 parts of diamond particles having an
average particle size of 150 nm in cyclohexanone and subjecting the
slurry to ultrasonic treatment was added to the above
dispersion.
1 Magnetic coating solution A: Hexagonal barium ferrite 100 parts
Surface covering compounds: Al.sub.2O.sub.3 5 mass %, SiO.sub.2 2
mass % Hc: 2,500 Oe (200 kA/m) Tabular diameter: 30 nm Tabular
ratio: 3 .sigma.s: 56 A .multidot. m.sup.2/kg Vinyl chloride
copolymer 6 parts MR110 (manufactured by Nippon Zeon Co., Ltd.)
Polyurethane resin 3 parts UR8200 (manufactured by Toyobo Co.,
Ltd.) Diamond particles 2 parts (average particle size: 150 nm)
Carbon black 1 part #50 (manufactured by Asahi carbon Co., Ltd.)
Isocetyl stearate 4 parts Stearic acid 1 part Oleic acid 1 part
Methyl ethyl ketone 80 parts Cyclohexanone 120 parts Magnetic
coating solution B: Hexagonal barium ferrite 100 parts Surface
covering compounds: Al.sub.2O.sub.3 5 mass %, SiO.sub.2 2 mass %
Hc: 2,500 Oe (200 kA/m) Tabular diameter: 30 nm Tabular ratio: 3
.sigma.s: 56 A .multidot. m.sup.2/kg Vinyl chloride copolymer 6
parts MR110 (manufactured by Nippon Zeon Co., Ltd.) Polyurethane
resin 3 parts UR8200 (manufactured by Toyobo Co., Ltd.) Diamond
particles 2 parts (average particle size: 150 nm) Isocetyl stearate
5 parts Stearic acid 1 part Oleic acid 1 part Methyl ethyl ketone
80 parts Cyclohexanone 120 parts Magnetic coating solution C:
Hexagonal barium ferrite 100 parts Surface covering compounds:
Al.sub.2O.sub.3 5 mass %, SiO.sub.2 2 mass % Hc: 2,500 Oe (200
kA/m) Tabular diameter: 30 nm Tabular ratio: 3 .sigma.s: 56 A
.multidot. m.sup.2/kg Vinyl chloride copolymer 6 parts MR110
(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin 3 parts
UR8200 (manufactured by Toyobo Co., Ltd.) Carbon black 1 part #50
(manufactured by Asahi carbon Co., Ltd.) Isocetyl stearate 5 parts
Stearic acid 1 part Oleic acid 1 part Methyl ethyl ketone 80 parts
Cyclohexanone 120 parts Nonmagnetic coating solution:
.alpha.-Fe.sub.2O.sub.3 hematite 100 parts Average long axis
length: 0.07 .mu.m Average short axis length: 0.014 .mu.m
S.sub.BET: 55 m.sup.2/g pH: 9 Surface covering compounds:
Al.sub.2O.sub.3 8 mass % Carbon black 25 parts Conductex SC-U
(average particle Size: 20 nm, manufactured by Columbia Carbon Co.,
Ltd.) Vinyl chloride copolymer 15 parts MR104 (manufactured by
Nippon Zeon Co., Ltd.) Polyurethane resin 7 parts (polyurethane
comprising neopentyl glycol/ hydroxycaproic acid/phthalic
acid/sodium salt of bis (2-hydroxyethyl) sulfoisophthalate/
diphenylmethanediisocyanate, mass average molecular weight: 40,000,
Tg: 38.degree. C., containg 6.5 .times. 10.sup.-5 eq/g of sodium
sulfonate) Phenylphosphonic acid 4 parts Isocetyl stearate 6 parts
Oleic acid 1.3 parts Stearic acid 1.3 parts Methyl ethyl
ketone/cyclohexanone 250 parts (8/2 mixed solvent)
[0138] Measurement of SNR:
[0139] SNR was measured with model RWA1001 disc-evaluating
apparatus (manufactured by Guzik Technical Enterprises, U.S.A.) and
Spin Stand LS-90 (manufactured by Kyodo Denshi System Co., Ltd.). A
signal of linear recording density of 100 KFCI was written with
complex MR head consisting of write track breadth of 1.5 .mu.m and
read track breadth of 0.9 .mu.m at an engine speed of 3,600 rpm and
the position of radius 44 mm. SNR value of the outermost periphery
was obtained from the reproduction output (TAA) and the noise level
after DC erasure, and the value obtained by measurement in the same
manner as above at the position of radius 22 mm was taken as SNR
value of the innermost periphery.
[0140] Measurement of Friction:
[0141] Friction was measured with RWA1001 disc-evaluating apparatus
(manufactured by Guzik Technical Enterprises U.S.A.) and Spin stand
LS-90 (manufactured by Kyodo Denshi System Co., Ltd.) carrying the
complex MR head used in the measurement of SNR connected to a load
cell (LVS-10GA, manufactured by Kyowa Electronic Instruments Co.,
Ltd., Japan). Frictional forces measured at an engine speed of
3,600 rpm and the positions of radius 22 mm and radius 44 mm were
respectively taken as Fin and Fout.
[0142] Measurement of Durability:
[0143] Durability was measured with RWA1001 disc-evaluating
apparatus (manufactured by Guzik Technical Enterprises U.S.A.) and
Spin Stand LS-90 (manufactured by Kyodo Denshi System Co., Ltd.)
using the complex MR head used in the measurement of SNR. The
positions from radius 22 mm to radius 44 mm were continuously
sought at an engine speed of 3,600 rpm. After starting seeking,
dropout in the sought area was measured every 10 hours, and the
time when a defect of the length of 300 .mu.m or more where dropout
lowered 30% or more was confirmed was taken as the lifetime of that
medium.
2 TABLE 1 Magnetic Coating Solution Di- Nonmagnetic Coating amond
Solution Average Fatty Fatty Particle Acid Ester Polyisocyanate
Acid Ester Polyisocyanate Sample Size Amt. Amt. Amt. Amt. No.
Remarks Kind (nm) Kind (pts.) (pts.) Kind (pts.) (pts.) 1 Comp. A
150 Isocetyl 4 3 Isocetyl 6 6 stearate stearate 2 Comp. A "
Isocetyl 6 " Isocetyl 8 " stearate stearate 3 Comp. A " Isocetyl 8
" Isocetyl 10 " stearate stearate 4 Ex. B " Isocetyl 4 " Isocetyl 6
" stearate stearate 5 Ex. B " Isocetyl " " Isocetyl " " stearate
stearate 6 Comp. B " Isocetyl 2 " Isocetyl 4 " stearate stearate 7
Ex. B " Isocetyl 6 " Isocetyl 8 " stearate stearate 8 Comp. B "
Isocetyl 8 " Isocetyl 10 " stearate stearate 9 Ex. B " n-Butyl 6 "
n-Butyl 8 " stearate stearate 10 Ex. B " n-Butyl 8 " n-Butyl 10 "
stearate stearate 11 Comp. A " Isocetyl 4 5 Isocetyl 6 10 stearate
stearate 12 Comp. A " Isocetyl " 8 Isocetyl " 16 stearate stearate
13 Comp. B " Isocetyl " 1 Isocetyl " 2 stearate stearate 14 Ex. B "
Isocetyl " 5 Isocetyl " 10 stearate stearate 15 Ex. B " Isocetyl "
8 Isocetyl " 16 stearate stearate 16 Ex. A 180 Isocetyl " 3
Isocetyl " 6 stearate stearate 17 Ex. A 200 Isocetyl " " Isocetyl "
" stearate stearate 18 Ex. C 150 Isocetyl " " Isocetyl " " stearate
stearate SNR Innermost Outermost Duration Sample Font Periphery
Periphery of Life No. Remarks Fin (mN) (mN) Fin/Fout (dB) (dB) (hr)
1 Comp. 40 20 2.0 22 20 300 2 Comp. 50 15 3.3 22 18 100 3 Comp. 60
10 6.0 22 14 20 4 Ex. 20 10 2.0 22 22 1,000< 5 Ex. 15 5 3.0 22
20 1,000< 6 Comp. 30 25 1.2 22 22 50 7 Ex. 25 10 2.5 22 22
1,000< 8 Comp. 45 5 9.0 22 12 300 9 Ex. 25 20 1.3 22 22
1,000< 10 Ex. 30 10 3.0 22 20 1,000< 11 Comp. 40 15 2.7 22 20
500 12 Comp. 40 10 4.0 22 19 500 13 Comp. 35 25 1.4 22 22 100 14
Ex. 25 10 2.5 22 20 1,000< 15 Ex. 20 5 4.0 22 19 1,000< 16
Ex. 30 20 1.5 22 22 1,000< 17 Ex. 25 15 1.7 22 21 1,000< 18
Ex. 20 15 1.3 22 22 1,000<
[0144] As can be apparent from the results shown in Table 1, by
restricting frictional force (Fin), frictional force (Fout) and
Fin/Fout to specific ranges, long duration of life can be obtained
in the samples according to the present invention, and high SNR can
be ensured in both of the innermost periphery and the outermost
periphery even with a track breadth as high as 1.5 .mu.m. On the
other hand, comparative samples cannot reconcile SNR with
durability.
[0145] This application is based on Japanese Patent application
JP2003-274091, filed Jul. 14, 2003, the entire content of which is
hereby incorporated by reference, the same as if set forth at
length.
* * * * *